The Collaborative International Dictionary of English v.0.48:

autism \autism\ n.
   1. (Med.) a mental disorder characterized by inability to
      engage in normal social interactions and intense
      self-absorption, and usually accompanied by other symptoms
      such as language dysfunctions and repetitive behavior.

   Note: autism usually manifests itself at an early age
         [PJC]

   2. behavior showing an abnormal level of absorption with
      one's own thoughts and disregard for external realities.
      [PJC]

WordNet (r) 3.0 (2006):

autism
    n 1: (psychiatry) an abnormal absorption with the self; marked
         by communication disorders and short attention span and
         inability to treat others as people